Heretofore, natural rubber has been applied to a wide variety of products ranging from industrial products such as tires for automobiles and aircraft, conveyer belts and adhesives to household products such as gloves. Natural rubber is collected as latex containing water, proteins, inorganic salts and the like in addition to the rubber component and this latex is coagulated to give crude rubber (crepe rubber or smoked sheet rubber). A target rubber product is produced from this crude rubber through mastication, addition of compounding agents, molding, and vulcanization.
Fresh latex of natural rubber contains non-rubber components such as proteins, lipids, carbohydrates and inorganic substances in addition to about 30 to 35% w/v (by weight/volume) of rubber component. Solid natural rubber (crude rubber) obtained by coagulating the fresh latex with formic acid contains about 6 wt % of the non-rubber components. These non-rubber components are known to be important for natural rubber to exhibit specific physical properties. However, it has become a social problem around 1990 that some of proteins contained in natural rubber latex products, gloves in particular, cause Type I acute allergy, and Food and Drug Administration (FDA) in U.S.A. has issued a medical alert toward manufacturers of rubber products to reduce soluble proteins contained in latex products.
Following methods are known as a technique for reducing proteins in latex, (i) a method of successive centrifuging the latex, (ii) a method of treating the latex with a proteolytic enzyme, (iii) a method of treating the latex with alkali, and (iv) a method of treating enzymatically deproteinized latex with alkali (refer to JP-A 30301 (2003)). However, even after treatments by these (i), (ii), and (iii) methods, the resulting natural rubber still contains a substantial amount of nitrogenous compounds and they are not able to be free from Type I allergic reactions. Moreover, the methods (ii) and (iv) using a proteolytic enzyme are resulted in the formation of rubber containing residual proteins originated from the enzyme, which can be some allergen. Furthermore, the method (iv) requires troublesome and costing two steps of treatment, i.e., the treatment of the proteolytic enzyme followed by alkali treatment and showed no finding to prove the formation of rubber free from allergy.
As a result of extensive studies on proteins contained in natural rubber latex, the present inventors have found that it is difficult to remove all proteins by ordinary proteolytic enzymes. Because, the proteins in the natural rubber are present in the serum of the latex and on the surfaces of rubber particles in the latex as shown in FIG. 1. The surface of the rubber particles in latex is known to be stabilized by a double-layer comprising of lipids and proteins.
Thus, by the method (i) of centrifuging the latex, the proteins in the serum can be removed, but the proteins on the surfaces of the rubber particles cannot be removed. Meanwhile, by the method (ii) of treating the latex with a proteolytic enzyme or the method (iii) of treating the latex with alkali, although the proteins on the surfaces of the rubber particles can be decomposed, coagulation of the rubber particles occurs during the treatment, so that enzymatic hydrolysis or chemical hydrolysis of the proteins remaining on the rubber particles becomes extremely slow, thereby making it impossible to remove all of the remaining proteins. Moreover, by the method (ii) the presence of residual proteins from the proteolytic enzyme is inevitable.
Under the circumstances, one of the present inventors has made intensive studies. As a result, he has found a method of producing natural rubber containing a reduced amount of nitrogen to 0.02% or less, the content of which is known to be an index of the protein content, and already filed a patent application for the invention (refer to JP-A 56902 (2003)). The method comprises the steps of treating natural rubber latex with a surfactant and a proteolytic enzyme and then concentrating and washing the treated latex once or twice by centrifugation. Since the latex obtained by this method has highly deproteinized, gloves prepared by use of the low-protein natural rubber reduce the possibility for Type I allergic reactions.
However, in this deproteinization method, the addition of about 1% of a surfactant or surfactants makes it difficult to produce solid natural rubber by an ordinary coagulation method. In addition, the method requires a step of removing decomposed proteins by centrifugation. Accordingly, the method is not suitable for mass production of natural rubber. Further, it has been confirmed by a clinical test with a scratch method, which is a more strict testing method, that about 8% of patients are still positive for Type I allergy with respect to the low-protein natural rubber obtained by this method (R. Hayakawa et. al., Environ. Dermatol., 6, 10 (1999)), indicating that deproteinization by this method is not complete.